The present invention relates to a phase-locked loop filter and, more particularly, to a digital phase-locked loop filter applicable to a motor speed control which is installed in an optical disk apparatus and others.
A digital phase-locked loop filter of the kind described is disclosed in, for example, U.S. Pat. No. 4,019,153. A problem with the loop filter disclosed in this patent is that the lockable frequency range thereof is too narrow to accommodate a speed signal outputted by a speed sensor, which is responsive to a motor speed or the like, and the frequency of which varies over a relatively wide range, failing to control motor speed with accuracy.